As I have followed the growth, development and recognition of this movement, it has become apparent to me that there is at least one aspect of body modification which has been essentially ignored by the main stream. This being the process of deliberate weight gaining. Perhaps it is only viewed as a particular sexual fetish, but my own experience with the subject leads me to believe that it is much more than that. Like any form of body modification, there is a great deal of variety in the number of approaches, goals and inspirations for gaining. As a gay male, I can only relate my experiences from a mostly male perspective, although, I would assume, many of the same principals may apply to females as well. Unfortunately, the female perspective is significantly more difficult to detail as there is a much greater bias against such practices.
In some cases, the gaining process it very specific. Many gainers focus their attention on increasing the size of their belly alone. Such males usually tend to be more dominant personality types with the distended belly becoming a symbol of power and authority. This, I believe, relates to many traditional authority figures such as the father or even such icons as the beer bellied police officer. Some examples I have encountered are no less extreme than any form of piercing or tattooing. There are individuals who have documented their progress and shown such a radical change in appearance as to become virtually unrecognizeable in relation to their initial form.
Others take a more general approach with the most extreme cases aiming for levels of obesity which would essentially leave them bed ridden and requiring constant care by others. Such individually seem to be quite the opposite of the "belly builder" and are usually submissive and desiring of a return to an almost infant like lifestyle where they are cared for and supported by their loved ones.
In both cases, the process of feeding can become quite ritualistic and methodical. Many individuals create very complex dietary guidelines for themselves and often incorporate elements of bondage into the feeding process with their partner assuming control of the subject's food intake. I have seen examples of this practice in primitive tribes where females are caged, kept immobile and fed for weeks or months in order to fatten them up for an impending wedding ceremony.
These examples represent some of the most extreme ranges for gaining. There are many subdivisions and lesser processes that I will not go into at this time. My only interest here is to outline the basic concept of gaining as a legitimate from of body modification and, as such, deserving of recognition within the greater canon of the art. If you are interested in learning more about this subject, I would refer you to GainRNews. You will find many examples of this practice along with excellent documentation.